The use of high voltage alternating current power distribution systems is accompanied by the risk that electrical maintenance personnel (and others) may inadvertently come into contact with energized conductors and be electrocuted or seriously injured.
In underground coal mines in the U.S., the high voltage electrical distribution systems are radial in design and they are operated at voltages typically in the range of 4,160 to 15,000 V a.c. In some special applications, distribution voltages may be lower and higher than that range. Power centers with transformers therein are located in active areas of the mine to transform the distribution voltage to suitable lower utilization voltages.
Because of the radial design of the underground high voltage distribution system, the high voltage sides of the power centers are typically of "feed-through" design. This means that the incoming high voltage supply cable is connected into the power center and loops directly out again to supply other power centers farther into the mine. An isolation switch on the primary side of each power center transformer can isolate the transformer from the high voltage supply, but the incoming and outgoing high voltage cables remain energized.
When the isolation switch on the primary side of any power center is closed to energize the transformer, it is immediately evident to anyone in the vicinity that the equipment is energized. The transformer produces the distinctly audible "mains hum" associated with the magnetizing of the core steel and also various indicator lights associated with the low voltage output circuits are illuminated.
In contrast, when the isolation switch is in the "open" position, there is no sound from the equipment, there are no indicator lights illuminated, and usually there is no voltmeter to indicate whether or not the incoming distribution voltage is available. In fact it is impossible to determine by visual or audeal inspection whether the incoming supply is on or off. This has been a contributory factor in numerous accidents.